1. Field of the Invention
This invention relates generally to code division multiple access (CDMA) systems and methods, and more particularly to a method and apparatus for decoder input scaling based on interference estimation in CDMA applications.
2. Description of the Prior Art
In code division multiple access (CDMA) cellular networks like W-CDMA for a universal mobile telecom system (UMTS) and CDMA2000 systems, the receive part of the user equipment (UE) consists of an inner receiver and an outer receiver. The inner receiver demodulates a received signal into symbols whereas an outer receiver forms a frame from symbols, performs frame processings such as frame interleaving, rate matching, and finally decodes error-correcting-coded frames. Among many possible techniques, a Rake receiver, a minimum mean squared error (MMSE) receiver, a decorrelating receiver, an equalizer and an interference canceller are widely used in CDMA systems. Because a wireless channel fades over time and the number of multipath components changes over time, the output of the inner receiver requires a large dynamic range. On the other hand, for the purpose of saving the frame memory, the output signal of the inner receiver is quantized to a smaller number of bits and (via some frame processing blocks like deinterleavers and rate matching blocks) goes to soft-decision decoders. It is well known that both Viterbi and turbo decoders require only 3 to 5 bits of input precision in an additive white Gaussian noise (AWGN) channel. But, in a mobile channel, because of the large dynamic range of the inner receiver, as many as 8 bits of precision have been used in some designs. This increases the memory requirement of the channel bit memory that dominates the entire UE modem memory requirement because of a large code block size.
In view of the foregoing, it would be both advantageous and desirable in the wireless communication art to provide a method and apparatus for adaptively scaling the decoder input for a low-cost UE digital baseband (DBB) chip. This method should reduce the memory requirement without degrading decoder performance.